LEDs are employed in a wide variety of applications. For example, in optical data transmission, LEDs are used to launch data signal alone a fiber-optic cable.
FIG. 1 depicts a prior-art AlGaInP quaternary LED. In the AlGaInP quaternary LED 100, a light-emitting region 110 is grown on the surface of an n-doped GaAs substrate 102. The light-emitting region 110 includes an n-doped AlGaInP layer 103, an AlGaInP active layer 104, a p-doped AlGaInP layer 105, and a p-doped GaP layer 106 arranged in the listed order. Moreover, a first electrode 108 is formed on the surface of the p-doped GaP layer 106 and a second electrode 109 is formed on the surface of the n-doped GaAs substrate 102. Typically, the AlGaInP active layer 104 is a double-heterostructure active layer or a quantum-well active layer.
Because the energy gap of the GaAs substrate 102 is less than the emission energy of the AlGaInP active layer 104, the GaAs substrate 102 will absorb some of the light generated within the AlGaInP active layer 104, thereby reducing the efficiency of the LED 100.
Improved performance can be achieved by employing an optically-transparent substrate instead of the n-doped GaAs substrate. The method is disclosed by the U.S. Pat. No. 5,502,316. Firstly, the removal of the n-doped GaAs substrate 102 is prior the formation of the electrodes. Next, an optically-transparent substrate 122 (e.g., n-doped GaP substrate, glass substrate, or quartz substrate) is bonded to the light-emitting region 110 at a relatively high temperature (e.g., 800˜1000□) utilizing a wafer-bonding technique. FIG. 2 depicts a LED 120 having an optically-transparent substrate 122 (e.g., n-doped GaP substrate), and the optically-transparent substrate 122 is electrically conductive. In the LED 120, the first electrode 108 is formed on the surface of the p-doped GaP layer 106 and a second electrode 111 is formed partially on the surface of the n-doped GaP substrate 122. Because the light generated in the AlGaInP active layer 104 can travel through the optically-transparent substrate 122, thereby enhancing the performance of the LED 120.
FIGS. 3A to 3F depict the steps of manufacturing a LED utilizing the prior-art wafer-bonding technique. In FIG. 3A, a single large-size substrate 102 is provided for the EPI process, wherein the substrate 102 is an n-doped GaAs substrate, also referred as a temporary substrate. In FIG. 3B, a light-emitting region 110 is formed on the surface of the substrate 102. In FIG. 3C, the temporary substrate 102 is removed and only the light-emitting region 110 is left. In FIG. 3D, a large-size permanent substrate 122 (e.g., optically-transparent substrate) is provided and wafer bonded to the light-emitting region 110 at a relatively high temperature. In FIG. 3E, a plurality of first electrodes 108 and a plurality of second electrodes 111 are formed on the surface of the light-emitting region 110 and the surface of the permanent substrate 122, respectively. At last, as depicted in FIG. 3F, a plurality of LEDs are manufactured after cutting the structure of FIG. 3E.
It is well understood that semiconductor material is easily to degrade at a relatively high temperature. Unfortunately, the wafer-bonding technique is necessarily processed at a relatively high temperature, and the relatively high temperature may degrade the light-emitting region 110. Moreover, because the sizes of the light-emitting region 110 and the permanent substrate 122 are relatively large, any uneven or particles adhered to the surfaces of the light-emitting region 110 or the permanent substrate 122 may fail the wafer-bonding step. Moreover, because the permanent substrate 122 is wafer bonded after the removal of the temporary substrate 102, the light-emitting region 110 would be unsupported by a substrate and will be difficult to handle without breaking.
Another method for fixing the light-absorbing problem in the substrate is disclosed by the U.S. Pat. No. 6,967,117 which adopts a reflecting layer for reflecting the light out the substrate. As depicted in FIG. 4A, a light-emitting region 110 is formed on the surface of a temporary substrate 102 (e.g., n-doped GaAs substrate), and the light-emitting region 110 sequentially includes an n-doped AlGaInP layer 103, an AlGaInP active layer 104, a p-doped AlGaInP layer 105, and a p-doped GaP layer 106. In addition, a buffer layer 145 and a reflecting layer 144 are sequentially formed on the surface of the light-emitting region 110. In FIG. 4B, a permanent substrate 142 is provided and a diffusion barrier layer 143 is formed on the surface of the permanent substrate 142. In FIG. 4C, the reflecting layer 144 is wafer bonded to the diffusion barrier layer 143 at a relatively high temperature, and then, a first electrode 112 is formed on the surface of the n-doped AlGaInP layer 103 and a second electrode 113 is formed on the surface of the permanent substrate 142 after the removal of the temporary substrate 102. Because the light upwardly toward the permanent substrate 142 will be reflected by the reflecting layer 144, thereby the performance of the LED 140 is enhanced.
FIGS. 5A to 5G depict the steps of manufacturing a LED utilizing the wafer-bonding technique disclosed in the U.S. Pat. No. 6,967,117. In FIG. 5A, a single large-size substrate 102 is provided for the EPI process, wherein the substrate 102 is an n-doped GaAs substrate, also referred as a temporary substrate. In FIG. 5B, a light-emitting region 110 is formed on the surface of the substrate 102, and then a buffer layer 145 and a reflecting layer 144 are sequentially formed on the surface of the light-emitting region 110. In FIG. 5C, a permanent substrate 142 is provided and a diffusion barrier layer 143 is formed on the surface of the permanent substrate 142. In FIG. 5D, the diffusion barrier layer 143 is wafer bonded to the reflecting layer 144 at a relatively high temperature. In FIG. 5E, the substrate 102 is removed from the structure of FIG. 5D. In FIG. 5F, a plurality of first electrodes 112 are formed on the surface of the light-emitting region 110 and a second electrode 113 is formed on the surface of the permanent substrate 142. At last, as depicted in FIG. 5G, a plurality of LEDs are manufactured after cutting the structure of FIG. 5F.
Alternatively, after the step depicted in FIG. 5E is completed, an etching procedure can be processed to partially remove the light-emitting region 110. A first electrode 112 and a second electrode 113 are respectively formed on the surface of the n-doped AlGaInP layer 103 and the portion of the p-doped GaP layer 106, and this structure is then cut into a plurality of planar-electrode LEDs as shown in FIG. 6.
In the above-described method, the wafer bonding is processed prior than the removal of the temporary substrate and the formation of the electrodes. However, even the problem resulted in the U.S. Pat. No. 5,502,316, a weak mechanical strength resulted by the removal of the temporary substrate, can be avoided in this method, a low reflectivity, so as reducing the efficiency of the LED is still resulted in due to an alloy procedure during the formation of the first and the second electrodes on the bonded chips. Moreover, the etching procedure processed to the light-emitting region 110 will reduce the surface area of the light-emitting region 110 depicted in FIG. 6, and current cannot uniformly travel through the light-emitting region 110, so as the efficiency of the LED is reduced.
The U.S. Pat. No. 6,221,683 discloses another method of manufacturing a LED. As depicted in FIG. 7A, a light-emitting region 110 is formed on the surface of a temporary substrate (e.g., n-doped GaAs), and the light-emitting region 110 sequentially includes an n-doped AlGaInP layer 103, an AlGaInP active layer 104, a p-doped AlGaInP layer 105, and a p-doped GaP layer 106. Next, a first metallic contacts layer 162 is formed on the surface of the n-doped AlGaInP layer 103 of the light-emitting region 110 after the removal of the temporary substrate. In FIG. 7B, a permanent substrate 166 is provided and on which a second metallic contacts layer 164 is formed. In FIG. 7C, a solder layer 163 is provided between the first metallic contacts 162 and the second metallic contacts 164, and the first metallic contacts 162 is wafer bounded to the second metallic contacts 164. Then, a first electrode 170 is formed on the surface of the p-doped GaP layer 106 and a second electrode 172 is formed on the surface of the permanent substrate 166, wherein the formation of the first electrode 170 and the second electrode 172 is not necessary after the wafer bonding step.
FIGS. 8A to 8G depict the steps of manufacturing a LED utilizing the wafer-bonding technique disclosed in the U.S. Pat. No. 6,221,683. In FIG. 8A, a single large-size substrate 102 is provided for the EPI process, wherein the substrate 102 is an n-doped GaAs substrate, also referred as a temporary substrate. In FIG. 8B, a light-emitting region 110 is formed on the surface of the temporary substrate 102. In FIG. 8C, a plurality of first metallic contact layers 162 are formed on the surface of the light-emitting-region 110 after the removal of the temporary substrate 102. In FIG. 8D, a permanent substrate 166 is provided and a plurality of second metallic contact layers 164 is formed on the surface of the permanent substrate 166. In FIG. 8E, a solder layer 163 is provided between the first metallic contact layers 162 and the second metallic contact layers 164, and the second metallic contact layers 164 are wafer bounded to the first metallic contact layers 162. In FIG. 8F, a plurality of first electrodes 170 are formed on the surface of the light-emitting region 110 and a second electrode 172 is formed on the surface of the permanent substrate 166. At last, FIG. 8G depicts that a plurality of LEDs are manufactured after cutting the above-described structure in FIG. 8F.
Similarly, the above-mentioned problems, including that the light-emitting region 110 is difficult to handle without breaking after the removal of the temporary substrate and the efficiency of the LED degrades during the alloy procedure, still occur.